ESCI 344 – Tropical Meteorology
Lesson 4 – Seasonal-mean Wind Fields
References: Forecaster’s Guide to Tropical Meteorology (updated), Ramage
Climate Dynamics of the Tropics, Hastenrath
Tropical Climatology (2nd ed), McGregor and Nieuwolt
Tropical Meteorology, Tarakanov
Climate and Weather in the Tropics, Riehl
General Circulation of the Tropical Atmosphere, Vol. II, Newell et al.
Reading: McGregor and Nieuwolt, Chapter 5 (e-reserve)
“Climatological Streamlines” (e-reserve)
“Climatological Streamlines (detailed)” (e-reserve)
“Tropical Cyclone Forecaster’s Reference Guide”, Chapter 2, Section 1.2 thru
1.5, (web resource) http://www.nrlmry.navy.mil/~chu
LOW-LEVEL WIND FIELD
January
Oceanic subtropical ridges merged with continental anticyclones.
Max easterly trade flow at about 10N.
Heat lows over Southern Africa.
In Atlantic and Eastern Pacific, trade-wind trough lies between 0 and 5N.
In Western Pacific and Indian Ocean, monsoon trough in SH, with northeast
monsoon firmly in place over Asia
April
Subtropical ridges close to January location.
African monsoon trough near 10N
In Atlantic and Eastern Pacific, trade-wind trough still lies between 0 and
5N.
Heat lows forming over China, Southern India, and Sahara Desert.
In Western Pacific and Indian Ocean, SH monsoon trough has moved
toward Equator.
A nearly symmetric trough forms in North Indian Ocean, and begins the first
of two tropical cyclone seasons in the Bay of Bengal and North Arabian Sea
July
African monsoon trough near 20N
Heat lows over Saudi Arabia and Mexico
Eastern Pacific now has a monsoon trough, instead of a trade-wind trough.
Monsoon trough sets in over South China sea and Western Pacific, and is
oriented northwest to southeast.
South Asia firmly entrenched in southwest monsoon flow, with monsoon
trough well inland.
October
Monsoon trough over North Africa is weak and lies near 10N.
Trade wind trough in Eastern Pacific.
Heat lows over Asia are gone.
Monsoon trough over South China Sea moves southward.
Monsoon trough draped across North Arabian Sea and Bay of Bengal,
leading to second tropical cyclone season in North Indian Ocean.
UPPER-LEVEL WIND FIELD
January
West Pacific and Indian Ocean under long ridges in both hemispheres.
Eastern Pacific and Atlantic have westerly flow, broken by anticyclone over
South America.
NH subtropical jet stream well developed, with strong maximum south of
Japan.
April
NH subtropical jet still present, but weaker.
SH subtropical jet moves toward Equator.
Ridges in both hemispheres have moved closer to equator.
Westerlies still dominate Eastern Pacific and Atlantic, but have pronounced
dip into SH.
July
Ridge over Asia has moved drastically northward.
NH subtropical jet has disappeared.
SH subtropical jet very strong near Australia.
Tropical easterly jet (TEJ) has formed over Asia.
Continuous ridge present in SH.
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Upper-level troughs, termed tropical upper-tropospheric troughs (TUTT) have
formed in North Pacific and North Atlantic.
Anticyclone established over Mexico.
October
Anticyclone in Asia-Africa region moved back toward Equator.
NH subtropical jet reestablished.
Anticyclone that was over Mexico has moved south.
Variable westerlies reestablish over Atlantic and North Eastern Pacific.
SH subtropical jet weakens somewhat.
SUBTROPICAL JET STREAM
The subtropical jet stream located in the upper troposphere above where the
Hadley cell descends.
In the SH the subtropical jet is present year-round.
In the NH the subtropical jet disappears in summer.
There are three speed maxima, over the Middle East, Asia, and America.
The subtropical jet is often very close to the polar jet.
Forecasters often refer to the subtropical jet as the “southern branch” of the
jet stream, though this is misleading as it implies that the subtropical jet is
just an offshoot of the polar jet.
The dynamics of the subtropical jet are very different than those of the polar jet.
The origin and maintenance of the subtropical jet is complex, and not completely
understood. Some significant factors are:
conservation of angular momentum as the air in the upper branch of the
Hadley cell moves poleward.
meridional gradients in the surface heat budget
Central Asian topography
Though there is no surface front or weather characteristics associated with the
subtropical jet, there is an upper-tropospheric front associated with it. This is
consistent with thermal wind balance.
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The gradients across the upper-tropospheric front are due in part to the
upper air convergence in this region as the upper branch of the Hadley cell
converges with the mid-latitude westerlies.
Though there is no surface weather associated with the subtropical jet, it does
interact with the midlatitudes and does play a role in midlatitude dynamics and
influences midlatitude weather.
TROPICAL EASTERLY JET
The tropical easterly jet (or TEJ) forms over Southern Asia and East Africa
during the boreal summer.
It exists because the mid-tropospheric thickness gradient is reversed due to the
intense heating over the Tibetan Plateau.
WEST AFRICAN MID-TROPOSPHERIC JET
The West African mid-tropospheric jet is found at about 600 mb, and extends
from the Red Sea to the Atlantic Coast.
Mean position is about 15N.
Strongest in boreal summer (~10 m/s).
Exists due to easterly thermal wind caused by heating over Sahara Dessert
contrasted with cooler, maritime air to South.
EAST AFRICAN LOW-LEVEL JET (SOMALI JET)
Found near 850 mb
Velocities 12-15 m/s.
African highlands are important geographic feature for creation and
maintenance of jet.
Maximum speeds in early morning, with minimum in early afternoon.
Likely due to changes in boundary layer stability between morning and
afternoon.
Important for mass transport and water vapor transport across Equator during
southwest monsoon.
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